This invention relates generally to methods and apparatus for the protection of electronic semiconductor circuitry against voltage surges. In particular, the invention relates to the protection against such voltage surges without relying on metal oxide varistors.
Many electronic circuits that employ semiconductor components require protection against voltage surges, particularly those carried on power lines. These so-called line borne voltage surges, which typically reach values in excess of 1000 V, can destroy semiconductor components. Surge protectors for such circuits conventionally employ metal oxide varistors ("MOV") for protection from voltage surges. For example, photo controls, which are widely used throughout the world for switching outdoor lighting, traditionally utilize metal oxide varistors for protection from line borne voltage surges. One such product, the 7570 series of electronic photo controls marketed by the Fisher Pierce Division of Pacific Scientific employs single or dual metal oxide varistors to suppress line transients.
The clamping voltage across an MOV is dependent on the current through it, and on its Joule rating. The Joule rating of an MOV is a measure of the transient energy that the MOV can dissipate without failure. For example, an MOV with a Joule rating of 160 and with a 320 V rating typically clamps the voltage at approximately 1000 V for a current of approximately 3000 A (ampere). The voltage across the MOV, however, can rise above 1000 V if the current increases further during a voltage surge. This excessive voltage can negate the protective capability of the MOV because the threshold voltage at which semiconductor components are damaged is normally about 1000 V.
The ability of an MOV to clamp the voltage across a semiconductor circuit is therefore limited by its Joule rating. Further, metal oxide varistors are not capable of providing protection against repetitive voltage surges separated by time intervals of less than a few minutes. This is due to their slow recovery, which typically is at least a few minutes after a surge. Given that repetitive line voltage surges are common, it is desirable to provide an apparatus and method for the protection of semiconductor circuitry against such surges.
The common failure mode of an MOV is understood to be the result of thermal overloading that occurs when an unusually high current passes through the device. This overload can cause a permanent short circuit which, in turn, increases the follow through current to the triggering threshold of a circuit breaker. The triggering of the circuit breaker then switches off the power to the semiconductor circuitry. It is, therefore, important to find alternative circuits for the protection of semiconductor circuits, including photo controls, against voltage surges without the limitations imposed by the intrinsic properties of an MOV.
Accordingly, it is an object of this invention to provide protection for an electronic circuit against line borne and other voltage surges, beyond the protection afforded by a metal oxide varistor.
It is another object of this invention to provide apparatus and method for the protection of semiconductor circuits against repetitive voltage surges, beyond the protection afforded by a metal oxide varistor.
The invention is next described in connection with illustrated embodiments. It will, however, be obvious to those skilled in the art that various modifications can be made to the embodiments without departing from the spirit or scope of this invention.